Emce Toys’ Make Your Own Zombie kit is a nice, but at $50-$60 for a single body and an assortment of heads, hands, etc., that’s a pretty expensive way to get into customizing action figures.
As an alternative, Classic TV Toys sells a variety of 8-inch Mego-style male and female bodies, heads and accessories that are reasonably priced. For example, you can grab a 5-pack of male bodies for $19. The site sells pretty much everything you’ll need to get started making your own custom figures.